This invention relates to spectrometry, and more particularly to ion mobility spectrometry.
Ion mobility spectrometry is an extremely sensitive method for detecting small quantities of vapor. Typically, the technique requires that the ambient air to be monitored be drawn over a dimethyl silicone membrane to separate the trace organic molecules to be detected from other atmospheric constituents such as water vapor etc. This enriched vapor is then passed into a small ionization chamber where the vapor is bombarded with either beta particles (typically from Ni.sup.63 source) or alpha particles (typically from an Am.sup.241 source). The ions thus formed are drawn into a drift tube by applying a brief electrostatic field across the ionization source. Once in the drift tube, the ions are accelerated by a linear electric field gradient applied along the length of the tube. Heavy ions achieve a slower drift velocity than lighter ions and are thereby separated. The ions reaching the end of the drift tube strike a metal collector electrode and the resulting ionic current is amplified by an electrometer. Thus, an ion mobility spectrum is produced by measuring the ion current versus time after admitting the ion pulse into the drift tube.
The ion mobility spectrometer is analogous, in some ways, to a time of flight mass spectrometer except that the ion mobility spectrometer operates at atmospheric pressure. Ion mobility spectra have much lower resolution and have peak positions which are dependent on the concentration of the ions as well as their type. Ion mobility spectrometers are extremely sensitive with part per trillion level detection limits for some compounds. Furthermore, they offer qualitative information about the constituents of the vapor being monitored.
Prior ion mobility spectrometers as described above, however, have one important weakness. The spectra produced are not always unique for a particular vapor because of the complex ion-molecule chemistry which can occur in the source prior to injection into the drift tube. Much of this problem is caused by the alpha or beta radioactive ionization sources which are not adequately selective in their operation. If a complex mixture of vapors is being analyzed then a very complex spectrum can result which may easily obscure the detection of the desired component. More particularly, in regard to the detection of toxic organophosphorus compounds, prior ion mobility systems can be confused by pollutant interference vapors present in high concentration (e.g. diesel exhaust) having the same molecular masses as vapors to be detected. thereby resulting in a high false alarm rate and unacceptable signal to noise ratios.